leds: lp5523: Support LED MUX configuration on running a pattern

There are two ways to run a pattern in LP5523.
One is using legacy sysfs files such as 'enginex_mode','enginex_load' and
'enginex_leds'. ('x' is from 1 to 3).
Among them, 'enginex_leds' are used for selecting specific LED channel MUX.
(MUX means which LEDs are used for running a pattern from LED 1 to 9.)

The other way is using the firmware interface.
In this mode, the default LED MUX strings are used.
In other words, LED MUX is not configurable on the fly.

This patch enables dynamic LED MUX configuration when the firmware is loaded.
By accessing the sysfs file 'enginex_leds', the LED channels can be configured.
To synchronize the operation mode, each engine mode should be set to 'LOAD'.

The documentation is updated as well.

Cc: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>